Articles prepared from foamed polymers offer several significant advantages over those made from unexpanded materials. For example, the insulation value per unit thickness of a foamed sheet is greater than that of a sheet of unfoamed polymer. Also, the strength-to-weight ratio of foamed materials is higher than unfoamed materials. These attributes have been applied to greatest advantage in the building trades where foamed polymer sheets are used as building insulation, and in the packaging area where foamed materials are used to fabricate lightweight trays and other food service products. Although polystyrene foam articles are the norm for applications which have less demanding thermal or mechanical requirements, there is a growing desire to successfully commercialize foamed articles obtained from other resins such as crystalline or semi-crystalline polyesters, primarily polyethylene terephthalate, ("PET"). Based on its resin properties, PET can be expected to provide better mechanical and thermal performance than polystyrene foam and better chemical and flame resistance. However, there are a number of well documented problems associated with the extrusion of polyester foam. In particular, we have observed instability and poor density reductions in polyester foam processes described heretofore. For example, U.S. Pat. No. 4,981,631 to Cheung et al. discloses foamed articles such as dual ovenable trays obtained from PET that contain 1 to about 6 wt. % of a polyolefin (i.e. polypropylene or polyethylene). Unfortunately, the maximum obtainable density reductions (about 50%) are generally considered unsatisfactory for many applications. We have also discovered that when a branching agent is used to improve the processing of the polyester, relatively high levels of the unreacted branching agent, for example pyromellitic dianhydride, remain in the finished article, which may be objectionable in the context of potential food uses or in the context of the long term stability of structural foam products sold into the housing market.
By way of background, the foaming of a resin via extrusion typically involves the steps of melting the polymer in the extruder, adding a foaming agent to the molten resin, then cooling the mixture in the forward sections of the extruder and forcing it through a die. Often this process is carried out in a pair of single screw extruders connected in tandem. The use of a twin screw extruder for the process is also well known. The foaming agent, usually a gas or a low boiling compound, is mixed with the molten resin in the extruder under sufficiently high pressure to maintain the resin/blowing agent mixture as a single phase within the extruder. Foaming occurs when this pressurized mixture exits the die, travelling from the region of higher pressure within the extruder, to a region of lower (usually atmospheric) pressure outside the extruder. The reduction in pressure causes the blowing agent to expand and form bubbles, thereby imparting a foamed quality to the extruded resin.
Substantial difficulties encountered in the extrusion of crystalline or semi-crystalline polyester foams (as distinguished from polystyrene foam) are caused primarily by: (1) a narrow "operating range" and (2) poor melt strength. The term "operating range" is recognized in the art as the optimum temperature for extruding the resin to produce a stable foam. At temperatures below the operating range, the molten polymer will either be too viscous to process, or if processable, too viscous to support foam cell growth, which means that any extrudate will have little if any foam character. At temperatures above the operating range the viscosity of the extrusion mass is low enough to permit expansion of the blowing agent gas, but too low to prevent the foam bubbles from collapsing, which also produces a poorly foamed, dense product. Ideally, the polymer to be foamed should have a very wide operating range. For example, in the case of polystyrene the difference between the highest and lowest temperature at which the resin can be successfully foamed is about fifty degrees centigrade. Generally speaking, the more slowly a polymer changes viscosity in response to a constant temperature gradient, the larger will be its operating range. In general, amorphous resins (e.g., polystyrene) have very large operating ranges because they typically elicit a gradual change in viscosity with temperature. On the other hand, semi-crystalline polymers such as poly(ethylene terephthalate) ("PET") exhibit a relatively abrupt transition from a low viscosity material above the crystalline melting temperature to a high viscosity polymer below the melting temperature. This permits a very narrow temperature region in which the polyester can be foamed. In the case of PET, the narrow operating range means that unless the extrusion melt temperature is very closely controlled, the foam will either form too easily upon extrusion of the resin from the die (and collapse on itself), or not form at all. The additional process cost associated with maintaining the extrusion conditions within PET's operating range is seen to be impractical.
Even if measures can be taken to maintain the extrusion process temperatures within PET's narrow operating range, a second problem noted above, the poor melt strength of the resin, impairs the ability of PET to support the growth of bubbles upon extrusion of the resin from the die. A number of patents disclose melt strength improvement of PET by reacting PET with a branching component. However, most of these patents do not discuss improvements in PET melt strength in the context of a foaming process. For example, Leslie et al. U.S. Pat. No. 4,145,466 discloses adding a polyanhydride (e.g. PMDA) to the PET in an amount from 0.1 to 5 wt. % based on the weight of the PET, preferably 0.2 to 1.5%, and most preferably 0.3 to 1.0 wt. %, to enhance blow molding and injection molding of PET. The manner in which the PMDA is added to the PET is described at column 3 lines 7-12, and Example 1 of the Leslie et al patent where it is stated that the PMDA and the PET are melt-mixed in an extruder. Although the patentee alludes to the manufacture of foams at column 3 line 38, no teachings of any foaming processes are given in the patent.
Another patent addressing the problem of PETs poor melt viscosity characteristics is Dijkstra et al U.S. Pat. No. 3,553,157. Example 4 and FIG. 1 of Dijkstra et al disclose mixing PMDA and PET where the amount of PMDA ranges from 0.5 to 1.0 wt % PMDA based on the weight of the PET. According to FIG. 1 of Dijkstra et al., the patentees' findings are that a maximum improvement in intrinsic viscosity is achieved at levels of PMDA of about 0.7 wt % based on the weight of polyester. Dijkstra et al. disclose that introduction of the polyfunctional compound (e.g. PMDA) can be achieved by masterbatching in which a mixture is prepared containing PET and the polyfunctional compound such that the mixture is very rich in polyfunctional compound. However, the patent does not state whether the masterbatch is prepared via extrusion or by simply dry mixing the PET and the polyfunctional compound. Dijkstra et al. patent does not deal with foam production.
A further patent dealing with melt strength improvement of PET is McCracken U.S. Pat. No. 4,933,429. Like the patents discussed above, McCracken does not discuss foam extrusion. The patentee discloses reaction of PET with about 0.05 to about 3.0 wt. % of a polyepoxide compound. In order to reduce exposure to patentee's preferred polyepoxide (triglycidyl isocyanurate, or "TGIC"), patentee states that it is preferred to initially prepare a concentrate by blending a relatively large amount of the TGIC with the polyester, where the amount of the TGIC in the polyester is in the range of about 3-20 wt % based on the weight of the concentrate. The concentrate is then blended with polyester to obtain a desired final level of TGIC in the polyester.
Turning now to patents which discuss polyester foam production, unexamined published Japanese patent application No. 59-210955 (1984), having a publication date of Nov. 19, 1984, discloses a method for manufacture of a thermoplastic polyester resin foam in which polyester resin (including PET) is mixed with 0.01 to 2 mole % (based on the PET) of a multifunctional carboxylic acid anhydride (including pyromellitic dianhydride "PMDA") and 0.03 to 2.5 wt. % of a multifunctional glycidyl ester. Although the method disclosed in the Japanese patent application concerns use of a PET resin composition containing (1) a multifunctional carboxylic acid anhydride and (2) a multifunctional glycidyl ester, FIG. 2 of the published Japanese application discloses the improvement in melt viscosity obtained when PET is combined with PMDA in the absence of the glycidyl ester.
Further patents dealing with polyester foam production are Hayashi et al. U.S. Pat. Nos. 5,000,991 and 5,134,028 which disclose a process for producing a thermoplastic polyester resin foam. The process of Hayashi et al. comprises melting a thermoplastic polyester resin (e.g. PET), mixing the molten resin with a blowing agent and extruding the mixture into a low pressure zone to carry out foaming. The process is characterized in that a compound having two or more acid anhydride groups per molecule (e.g. PMDA) is added to the thermoplastic polyester resin. Hayashi et al. '991 disclose using an amount of PMDA in the range of from about 0.05 to 5.0 parts by weight per hundred parts by weight of the thermoplastic polyester resin. The patentees teach at column 5, lines 10 to 13, that "when the amount [of PMDA] exceeds 5.0 parts by weight, the gelation of the molten material of the thermopolastic polyester proceeds and extrusion foaming cannot be effected". Hayashi et al. '991 state at column 6, lines 36 to 61, that the thermoplastic polyester resin can be mixed with the compound having two or more acid anhydride groups (i.e. PMDA) in any of three possible methods: First, by mixing (without melting) the polyester resin pellets with PMDA powder to coat the pellets with PMDA; or, secondly, by pre melt-mixing the PMDA with a thermoplastic resin (which can be the same or different from the polyester), pelletizing the mixture, and adding the pelletized mixture to the polyester; or, thirdly, by melting the polyester in the extruder and then adding the PMDA to the extruder to effect mixing. The patentees equate these methods and do not suggest or teach any different outcome in the foaming process if one mode of combining the PET and PMDA is chosen over another. Hayashi et al. '991 also disclose the addition of a compound of a metal of Groups I, II or III elements of the periodic Table (e.g. sodium carbonate) to the polyester resin in an amount of 0.05 to 5 parts by weight per hundred parts of polyester resin. The patentee states that the metal compound results in foams having higher tensile elongation and finer cells. Hayashi et al. '991 teach against using levels of sodium compounds in the process below about 220 ppm.
Notwithstanding the disclosure of Hayashi et al., we have observed instability in the process for extruding a low density polyester foam wherein PET, PMDA, (and optionally sodium carbonate) and a foaming nucleator, such as talc, are fed into either a twin screw extruder or a 3/4" single screw extruder. In particular, we observed large variations in product quality during runs of more than an hour. Instability of the process was indicated by the fact that the extruder torque and pressure were observed to double or halve without any change in process settings. The inherent viscosity of samples exhibited similar wide fluctuations. These factors resulted in PET foams with large density and microstructure inconsistencies. Moreover, we were unable to attribute these inconsistencies to feed variations, moisture effects or equipment variations. In addition we have determined that the process of Hayashi et al. '991 results in amounts of unreacted PMDA which it is desired to reduce.
A further problem in the extrusion foaming of polyester foam is the difficulty in using recycled PET as a feedstock for such foaming. Generally, the inherent viscosity of recycled PET is lower than virgin PET due to processing performed by recycling processors to remove impurities. A problem encountered in attempting to use recycled PET is that there are wide differences in the extent to which different lots or sources of PET recycle can be improved in melt viscosity via chain branching reaction with PMDA. Some lots of recycled PET exhibit very good improvement in melt viscosity when reacted with a chain branching agent such as PMDA, while others do not. We have sought to overcome this problem so that a manufacturer of foamed PET articles can consistently and reliably employ PET from all possible sources, including recycle.
Still another problem to overcome in PET melt processing is that extruder throughputs, while generally satisfactory, should be higher in order to render such processes more economical. The branching reaction between PMDA and PET, while improving the processability of PET, has a tendency to cause increases in the extruder torque near the beginning of the extrusion line which can hamper throughput. For example, when PET is extruded with PMDA to obtain a melt strength enhanced resin, a subsequent remelting of this already extruded material to melt fabricate articles typically encounters high viscosities at the beginning of the extrusion line, hence reducing throughput. It is desired to minimize this problem as much as possible so that extruder throughput can be increased.